1. Field of the Invention
The present invention relates to a power generator, power generation system and wireless power transmission system for transferring energy through the air.
2. Description of the Related Art
To avoid exhausting valuable natural resources and to cope with the global warming effect that has been growing year by year, people have been paying more and more attention to solar power generation that never releases harmful carbon dioxide. In fact, a power plant for generating a lot of electric power by installing a huge number of solar power generators (i.e., so-called “solar cells”, which will be sometimes simply referred to herein as “cells”) in a wide area has become a reality. In most cases, solar power generators for general consumers have been installed on the roof of a house. Recently, however, some people have proposed putting those cells on the walls of a building, too.
A solar power generation system ordinarily uses a so-called “solar cell module” in which a huge number of cells are arranged inside a metallic frame and connected together. A glass plate is arranged in front of the solar cell module (which will be simply referred to herein as a “module”) so that the respective cells operate without being exposed to the air. And by assembling a number of such solar cell modules together, a solar power generation system can be established.
Such a solar power generation system, however, has not been widespread yet because the cost of manufacturing those cells and modules is too high, which is one of the major obstacles to its introduction. On top of that, the cost of establishing such a system by installing those cells and modules is also too high to ignore. Among other things, the higher the altitude of the place of installation, the riskier and the more expensive the installation work will be, which is a serious problem to overcome in order to further popularize the solar power generation system. What is more, to introduce a solar power generation system into a non-new building, it is difficult to install to connect the solar power generating section outside of the building to electronic devices inside of the building, which is also one of the big problems with conventional solar power generation systems.
As will be described later, in a conventional solar power generation system, the output voltage of each of its cells is so low that a great many solar cells should be connected together to obtain a voltage that is high enough to operate an electronic device. And a decrease in reliability at such a huge number of connection points is a decisive factor in the decline of the long-term reliability of the overall system. In addition, if those modules and cables get deteriorated with a long-term use, their replacements should also be installed at such a height. Consequently, the cost of maintenance is non-negligible.
As a conventional solar power generator that would overcome such problems, a power supply system for supplying energy wirelessly from outside of a building and through the walls of the building has been proposed (see Japanese Patent Application Laid-Open Publication No. 2006-136045 (Embodiment 5 and FIG. 16), for example). Such a power supply system transmits RF (radio frequency) energy through the walls by electromagnetic induction.
Meanwhile, a power generation system that uses a fuel cell such as a polymer electrolyte fuel cell has also become more and more popular these days. In a power generation system of that type, the output voltage of each of its cells is also so low that a great many cells should be connected together to obtain a sufficiently high voltage. That is why as in the solar power generating device, a decrease in reliability at such a huge number of connection points is a decisive factor in the decline of the long-term reliability of the overall system.
On the other hand, United States Patent Application Publication No. 2008/0278264 (FIGS. 6 and 11) discloses a new type of wireless energy transfer system for transferring energy from one of two resonators to the other, and vice versa, through the air between them. That wireless energy transfer system couples those two resonators with each other via the evanescent tail of the oscillation energy of the resonant frequency that is produced in the space surrounding those two resonators, thereby transferring the oscillation energy wirelessly (i.e., by a non-contact method). Such an energy transfer method in which a magnetic field distribution is used by resonators will be referred to herein as “resonant magnetic coupling method”.
According to United States Patent Application Publication No. 2008/0278264, the wireless power transmission by the resonant magnetic coupling method should increase the transmission distance significantly compared to the conventional electromagnetic induction method. That is to say, if the coupling coefficient k between the resonators is greater than the root of the product of the respective attenuation constants ┌1 and ┌2 of the two resonators, energy should be transferred as intended.
The power supply system disclosed in Japanese Patent Application Laid-Open Publication No. 2006-136045, however, cannot overcome the solar power generation device's own problem that the output voltage of each cell is low. In the field of solar power generation, a crystalline silicon based solar cell, which is currently used broadly due to its high energy conversion efficiency, has an output voltage Vc of just about 0.5 V. For example, if the DC output of a solar power generating section needs to be converted into AC power, the operation efficiency of a normal power conditioner is maximized in response to an input voltage of approximately 300 Vdc. That is why to get that conversion done with high efficiency, the output voltage of the solar power generating section should be increased to the vicinity of 300 V by connecting as many as several hundreds of cells together in series. On the other hand, if connected to a single-phase, three-wire grid (with a working voltage of 100 V or 200 V), which is a normal household wiring system, the solar power generating section may have its output voltage increased by a power conditioner as much as 200 fold or more. Considering the decrease in power efficiency to be caused by increasing the voltage that much, it is still preferred that a huge number of cells be connected together in series to increase the output voltage of the solar power generating section as much as possible.
It should be noted that even if the DC voltage is not converted into AC power within such a solar power generation system, a similar problem will also arise. For example, in a DC power supply system that has attracted a lot of attention these days, its working voltage will be either 48 Vdc or within the range of 300 to 400 Vdc. That is why even when solar energy needs to be supplied to a DC power supply system, several tens to several hundreds of solar cells also need to be connected together in series.
However, the greater the number of cells or modules to be connected together in series, the more easily the overall performance of the system will decline due to either so-called “partial shading” (i.e., some of the installation zone goes into the shade) or deterioration in the property of some of those cells or modules to be installed. To overcome such a problem, normally a countermeasure such as introduction of a bypass diode into each module is taken. Such a measure is not preferred because an excessive quantity of heat will be generated or the cost will rise significantly in that case. Meanwhile, even when the voltage needs to be increased using a normal DC/DC converter with a voltage boosting function, it is also difficult to achieve a voltage step-up ratio that is high enough to significantly reduce the number of cells to be connected together in series.
On the other hand, when wireless power transmission is carried out by the resonant magnetic coupling method, the output impedance Zoc of RF energy to be output from an oscillator is preferably substantially matched to the input impedance Zin of a power-transmitting antenna with the output terminal of a power-receiving antenna connected to a load. Also, the output impedance Zout of the power-receiving antenna is preferably substantially matched to the resistance value R of the load connected to the power-receiving antenna with the oscillator connected to the power-transmitting antenna. These conditions are preferably satisfied in order to minimize multiple reflection of RF energy between circuit blocks and to increase the overall power generation efficiency.
However, the output impedance of a solar cell may vary according to some environmental condition such as the intensity of the sunlight that irradiates the cell and the temperature of the cell itself. For that reason, if a fixed load is driven by a solar cell, mismatch may be caused between the output impedance of the cell and the impedance of the transmission line due to a variation in the intensity of the sunlight received or the temperature of the cell itself. As a result, the power transmission efficiency will decrease.
Likewise, the output impedance of a power generation system using a fuel cell also varies according to some environmental condition on its power generating section. Specifically, the output impedance of the power generating section may vary with the pressure of hydrogen gas injected or the temperature of the cell. In any case, it is difficult to maintain stabilized energy output under those environmental variations.
It is therefore an object of the present invention to provide a power generator and power generation system that can not only raise the ordinarily low output voltage of the power generating section significantly but also maintain stabilized energy output even under some environmental variations in the intensity of the sunlight received or its own temperature. Since the power can be transmitted wirelessly according to the present invention, the installation work and replacement of some cells or modules can get done much more easily.
The present invention is applicable to not just a solar power generation system but also a fuel cell power generator that uses a polymer electrolyte fuel cell, for example. According to the present invention, the ordinarily low output voltage of a power generating stack can be raised significantly on a cell-by-cell basis. Added to that, stabilized energy output can be maintained even if the pressure of hydrogen gas supplied or the environmental temperature varies. On top of that, even if any of those cells has gone out of order, such a defective cell can be replaced very easily according to the present invention.